At 11.06 2005.03.07 -0600, you wrote:
>On Fri, 4 Mar 2005 14:46:49 -0500, Vlad <vlad@doom.net> wrote:
>> On Fri, Mar 04, 2005 at 11:46:59AM -0500, Dave VanHorn wrote:
>> > >In the case of smaller PICs, such as the 16f628, Vdd and Vss are at
>> > >opposite ends of the chip - symmetrically, right across from each other.
>> > >
>> > >Would it make sense to put the cap hidden inside a chip socket (if a
>> > >socket is used in the design) with one lead to Vss and the other to Vdd?
>> > >
>> > >I think that configuration would give you the shortest trace possible.
>> 
>> This is what I had in mind:
>> 
>>        Vss ----------++---------- Vdd 5v
>>                      ||
>>                 +----++----+
>>                 |1   ||  18|
>>                 |    ||    |
>>                 |    ||    |
>>                 |    ||    |
>>                 |5---)|--14|
>>                 |   -  +   |
>>                 |   0.1uF  |
>>                 |  ceramic |
>>                 |9       10|
>>                 +----------+
>>                    16F628
>> 
>> Or maybe even:
>> 
>>           Vss |              | Vdd 5v
>>               |              |
>>               | +----------+ |
>>               | |1       18| |
>>               | |          | |
>>               | |          | |
>>               | |          | |
>>               +-+5---)|--14+-+
>>                 |   -  +   |
>>                 |   0.1uf  |
>>                 |  ceramic |
>>                 |9       10|
>>                 +----------+
>>                    16F628
>> 
>> > You do, but even then, you can get better.
>> >
>> > What you want, is for VCC to go to the cap, and then to the chip.
>> > You don't want VCC to go across the top of a circuit that is the cap, plus
>> > lead inductance.
>> >
>> > Imagine a "T" where VCC enters on the left, and the chip connects on the
>> > right.
>> > The cap is at the bottom. This puts almost all the lead inductance in place
>> > making the bypass cap less effective.
>> >  The ideal connection is a "V" where you force ALL the current to pass the
>> > cap.
>> > Now the lead inductances form a "tee filter" and actually help you.
>
>You guys really like to over-engineer stuff.  A 0.1 uF chip reasonably
>near the power pins should take care of almost any circuit a PIC would
>be involved in.  For extra decoupling, put 10 ohms in series with the
>Vdd supply (before the capacitor). Don't worry about custom chip
>sockets etc.  You Ain't Gonna Need It (tm).

You like to miss-engineer stuff. Better yet a ferrite bead. ;->


>Regards,
>Mark Rages
>markrages@gmail
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